There is a continuous effort to develop pressure sensors that are lower in cost and smaller in size, yet are characterized by high reliability, sensitivity and linearity. Sensors finding wide acceptance on the basis of furthering these characteristics include those that utilize semiconductor materials with a micromachined sensing diaphragm, a notable example being micromachined single-crystal silicon pressure transducer cells manufactured using semiconductor fabrication processes. In the processing of such cells, a thin diaphragm is formed in a silicon wafer through preferential chemical etching. Ion implantation and diffusion techniques are then used to drive doping elements into the diaphragm, forming piezoresistive elements whose electrical resistance changes with strain (this ratio being termed the "gage factor"). As a result, deflection of the diaphragm causes a change in resistance value of the piezoresistive elements, which can then be correlated to the magnitude of the pressure applied to the diaphragm.
Diaphragms of single-crystal silicon pressure transducer cells are typically small, rarely exceeding a few millimeters in width, and are very thin, with a thickness of often less than 100 micrometers. The use of standard single-crystal silicon wafers and standard semiconductor device fabrication processes allows many such cells to be fabricated from a single wafer, providing some economy of scale. However, silicon is susceptible to chemical attack and erosion by various media, particularly in applications where a high-pressure medium is to be sensed, e.g., automotive applications that involve sensing brake fluid, oil, transmission fluid, hydraulic fluid, fuel and steering fluid pressures. For such applications, a pressure sensor must also be physically rugged and resistant to the hostile environment of the sensed medium, necessitating that a micromachined silicon pressure transducer cell include some form of protection in order to realize its advantageous operational characteristics in the chemically hostile environment. Current methods for producing media-compatible, high-pressure sensors include enclosing a silicon sensing chip in an inert fluid, such as a silicone oil or gel, and then further separating the sensing chip from the medium to be sensed with a metal diaphragm, such that pressure must be transmitted through the metal diaphragm and fluid to the sensing chip. While achieving some of the operational advantages of silicon pressure transducer cells, the manufacturing processes for these sensors are relatively expensive and complicated. As a result, these sensors are not suitable as mass-produced sensors for automotive applications.
As known in the art, alternative approaches include ceramic capacitive pressure sensors and ceramic diaphragms that use thick-film piezoresistors as strain sensing elements. However, each of these also have certain disadvantages, such as complex circuitry to detect capacitance changes, the requirement for ceramic-to-ceramic bonds, and a maximum pressure capability typically not exceeding about 1000 psi (about 7 MPa). For higher pressures, metal diaphragms have found use as the sensing element. Because metal diaphragms generally deflect more for a given thickness and pressure than ceramic diaphragms, sensing is performed by thin-film polysilicon or metal deposited on the metal diaphragm. The diaphragm must first be coated with a dielectric layer to electrically insulate the diaphragm from the thin-film resistors and conductors. A thin-film polysilicon layer is then deposited to form the piezoresistors, followed by thin-film metallization to provide electrical interconnects. As is conventional, the thin-film layers are typically deposited by such processes as chemical or physical vapor deposition. The equipment necessary for these processes is expensive, and deposition rates are extremely slow. Deposition of the thin-film layers requires multiple patterning, exposure, developing and stripping steps for the required thin-film photoresists and metallization, and must be carried out in a controlled environment to assure that no air borne particles are present on the surface to be coated. In addition, because such processes deposit thin-films usually no thicker than 10,000 angstroms, the surface of the metal diaphragm must be extremely smooth to avoid rough surface features penetrating through or producing discontinuities in the deposited thin films. Finally, the resistance of the resulting polysilicon thin-film piezoresistors can vary dramatically with temperature.
While achieving some of the operational advantages of silicon pressure transducer cells, metal diaphragm pressure sensors of the type described above have complicated manufacturing processes that render the sensors incompatible with mass-production applications. Media-compatible, high-pressure transducer cells that combine a corrosion-resistant metal diaphragm and thick-film piezoresistors have been proposed, as taught in U.S. patent application Ser. No. 08/954,266 now U.S. Pat. No. 5,867,886 to Ratell et al. (Attorney Docket No. H-198882). Such sensors are capable of sensing very high pressures while being chemically and mechanically robust, readily manufacturable, and relatively insensitive to temperature variations. At least one thick-film dielectric layer is required to electrically insulate the metal diaphragm from the thick-film piezoresistors. For compatibility with the metal diaphragm, the dielectric layer applied directly to the diaphragm must be formed of a material that will adhere to the metal diaphragm, withstand the strains induced as the diaphragm deflects, faithfully transmit such strains to the thick-film piezoresistors, and compensate for the coefficient of thermal expansion (CTE) mismatch between the metal diaphragm and piezoresistors. A complication is that metal oxide constituents of dielectric materials found suitable for this purpose have been found to diffuse into the thick-film piezoresistor and react with metal oxides present in the frit component of the piezoresistor, thereby significantly increasing the sheet resistivity of the piezoresistor, e.g., above the 3 to 10 kilo-ohm/square (k.OMEGA./.quadrature.) range typically desired. Accordingly, improved performance could be achieved if diffusion between the electrical-insulating layers and the thick-film piezoresistors was inhibited or at least controlled. While there have been suggestions to compensate CTE mismatch with temperature compensation electronics instead of manipulating the composition of the required insulating material, doing so undesirably increases processing and costs of the sensor.